Investigations on Large Cosmic-Ray Bursts

Observations on large cosmic-ray bursts have been performed at the altitude of 3500 meters with a model C meter surrounded by thick lead shields. G-M counters, connected to appropriate electronic circuits, were placed over these lead shields in order to record extensive air showers simultaneously with bursts. It is found that about 2 percent of the 100 particle bursts and about 14 percent of the 1000 particle bursts under 10.7 cm Pb are produced by extensive air showers. This fraction continues to increase with increasing burst size and becomes almost 100 percent for bursts of more than 4000 particles. The experimental observations are consistent with the assumption that the part of the burst-producing radiation which does not consist of air showers or of $\mu$-mesons is attenuated in lead with the absorption mean-free-path of 346±36 g/${\mathrm{cm}}^2$. The correction for the number of bursts produced by $\mu$-mesons is based on a revision of the calculations of Christy and Kusaka; the revised calculation indicates that $\mu$-mesons (spin 1/2) produce 68±5 percent of the bursts in a model C meter under 10.7 cm Pb at sea level.